The “flagship of the Royal Navy” is to be decommissioned after a multi-million pound refit.
The Devonport-based helicopter carrier and assault ship underwent a £65m upgrade in 2014.
The MoD said it would maintain a “significant amphibious capability”.
It said there would be new Type 26 warships and frigates; two Queen Elizabeth-class aircraft carriers; as well as the existing fleet which includes HMS Bulwark.
When making the announcement the MoD said it was old news.
However, in 2012 ahead of the £65m refit, Defence Minister Philip Dunne said: “I am delighted that this contract will not only ensure that HMS Ocean remains a significant, highly-flexible and capable warship for years to come”.
BBC South West political editor, Martyn Oates, said the announcement raised concerns for its home port of Plymouth.
“With no like-for-like replacement planned, concerns in Ocean's home port of Plymouth will focus on how many of the new ships promised by the Prime Minister yesterday will be based in the city and whether that will be enough to fill the rather large hole left by the retiring flagship.”
An MoD spokesman said: “HMS Ocean will not decommission early and will continue in service as planned well into this Parliament.
“As part of the SDSR process, the decision was taken not to extend her and to decommission her in 2018, in line with her 20-year life span.”
HMS Ocean – Britain's largest warship – is currently deployed in the Mediterranean on a Nato exercise.